I think the documentation is correct there, because in the example they replace from the middle of the name, e.g.: frame_normal_1.png, so to make it fame1.png they used _normal_ pattern to be replaced with nothing.
Analogically, in my case images are named platform_normal.png, so to make platform.png I use _normal pattern to be replaced with nothing.
So _ is the actual sign to be replaced.
EDIT:
Plus in Editor - when you set a correct rename pattern, you see the change in Outline:
Correct (name of image is “platform”) vs incorrect (name of image is “platform_heightmap”, because of typo in Rename Pattern):
Yup you are right ! I took a quick look and tint seems to be the problem here, changing it to a positive value will show your results and you may want to bring the go with with sprite into view a little more as it is sitting in the bottom left corner.